I've researched this forums and found no similar problem to mine. Can some please advise. Both my Central AC and Heater will not respond to my thermostat when I turn to Auto. However, when it's in the ON position, the fan blows cool air, no heat or AC. Do I need a new thermostat? Thanks in advance.

Tom

Your thermostat is probably OK but

The heat fan control on the furnace could be defective
The relay for the A/C fan control could be defective
The low voltage wiring fot the furnace to the thermostat could be hooked up incorrectly.

There are many reasins for the and the one that happens more often than not is the fact tha a person who installed the system or added A/C at a later date did not know what they were doing BUT if this just started to happen it will probably be one of the items mentioned above.

Thanks for the quick reply, but is it common for both the AC relay and the fan control to go out at the same time? Both were in working condition last week, but then now neither will come on in the "Auto" position.

If the fan relay is being used for both items then it is possible. It is an item that should be checked and tested before any replacements of parts are made.